Aligned polymer organic TFT

ABSTRACT

A method for forming an electronic device having a semiconducting active layer comprising a polymer, the method comprising aligning the chains of the polymer parallel to each other by bringing the polymer into a liquid-crystalline phase.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of application Ser. No. 10/018,425 filedDec. 20, 2001; the disclosure of which is incorporated herein byreference.

[0002] This invention relates to aligned polymers, especially alignedpolymers suitable for use in devices such as polymer thin filmtransistors, and methods of aligning polymers. The aligned polymers arepreferably substantially parallel aligned, liquid-crystalline conjugatedpolymers.

[0003] Semiconducting conjugated polymer field-effect transistors (FETs)have potential applications as key elements of integrated logic circuits(C. Drury, et al., APL 73, 108 (1998)) and optoelectronic devices (H.Sirringhaus, et al., Science 280, 1741 (1998)) based on solutionprocessing on flexible plastic substrates. One main criterion to obtainhigh charge carrier mobilities has been found to be a high degree ofstructural order in the active semiconducting polymer.

[0004] For some polymers it is known to be possible to induce uniaxialalignment of the polymer chains in thin films by using processingtechniques such as Langmuir-Blodgett (LB) deposition (R. Silerova, Chem.Mater. 10, 2284 (1998)), stretch alignment (D. Bradley, J. Phys. D 20,1389 (1987)), or rubbing of the conjugated polymer film (M. Hamaguchi,et al., Appl. Phys. Lett. 67, 3381 (1995)). Polymer FET devices havebeen fabricated with uniaxially aligned polymer films fabricated bystretch alignment ((P. Dyreklev, et al., Solid State Communications 82,317 (1992)) and LB deposition (J. Paloheimo, et al., Thin Solid Films210/211, 283 (1992)). However, the field-effect mobilities in thesestudies have been low (<10⁻⁵ cm²/Vs).

[0005] Local order in thin polymer films can be achieved by making useof the tendency of some polymers to self-organise. An example ispoly-3-hexylthiophene (P3HT) in which lamella-type ordered structurescan be formed by phase segregation of rigid main chains and flexibleside chains. By using suitable deposition techniques and chemicalmodification of the substrate it is possible to induce preferentialorientations of the ordered domains of the polymer with respect to thesubstrate surface. At present P3HT yields the highest known field-effectmobilities of 0.05-0.1 cm²/Vs for polymer FETs (H. Sirringhaus, et al.,Science 280, 1741 (1998)). In these known devices there is nopreferential, uniaxial alignment of the polymer chains in the plane ofthe film.

[0006] Some conjugated polymers and small molecules exhibitliquid-crystalline (LC) phases. By definition, a liquid-crystallinephase is a state of matter, in which the molecules have a preferentialorientation in space. This alignment is conventionally regarded as beingalignment with respect to a vector called the director. Unlike in thesolid, crystalline state the positions of the molecules in the LC phaseare randomly distributed in at least one direction. Depending on thetype of orientational and residual positional order one distinguishesbetween nematic, cholesteric and smectic LC phases. The nematic phasepossesses long-range orientational order but no positional order.Smectic phases are characterized by a two-dimensional (2D) layeredstructure, in which the molecules self-assemble into a stack of layerseach with a uniform orientation of the molecules with respect to thelayer normal, but either no positional order or a reduced degree ofpositional order in the 2D layers. LC phases occur mainly inpolymers/molecules with a significant shape anisotropy. Examples ofconjugated LC polymers are main-chain polymers with a rigid-rodconjugated backbone and short flexible side chains, so-called hairy-rodor rigid-rod polymers. Examples are poly-alkyl-fluorenes (M. Grell, etal., Adv. Mat. 9, 798 (1998)) or ladder-type poly-paraphenylenes (U.Scherf, et al., Makromol. Chem., Rapid. Commun. 12, 489 (1991)). Anothertype of LC polymers are side-chain polymers with a flexiblenon-conjugated backbone and rigid conjugated units in the side chains.

[0007] A special class of liquid-crystalline organic molecules aredisc-shaped molecules with a rigid 2D conjugated core and flexible sidechains such as hexabenzocoronenes (HBC) (P. Herwig, et al., Adv. Mater.8, 510 (1996)) or triphenylenes (D. Adam, et al. Nature 371, 141(1994)). They tend to form so-called discotic mesophases in which1-dimensional columns are formed by π-π stacking of the disc-shapedconjugated cores (FIG. 8).

[0008] LC phases typically occur at elevated temperatures in theundiluted organic material (thermotropic phases) or if the organicmaterial is dissolved in a solvent at a sufficiently high concentration(lyotropic phases) (see, for example, A. M. Donald, A. H. Windle, LiquidCrystalline Polymers, Cambridge Solid State Science Series, ed. R. W.Cahn, E. A. Davis, I. M. Ward, Cambridge University Press, Cambridge, UK(1992)).

[0009] LC polymers can be uniaxially aligned by suitable processingtechniques. In an aligned sample the orientation of the director, thatis, for example, the preferential orientation of the polymer chains in amain-chain LC polymer, is uniform over a macroscopic distance of >μm-mm.This is the scale of practical channel lengths in FET devices. Alignmentcan be induced by shear forces or flow or by depositing the LC polymeronto a substrate with an alignment layer exhibiting a uniaxialanisotropy in the plane of the substrate. The alignment layer may be amechanically rubbed organic layer such as polyimide (M. Grell, et al.,Adv. Mat. 9, 798 (1998)), a layer evaporated at an oblique angle ontothe substrate, or a layer with a grooved surface. For a review of thevarious techniques which can be used to align LC molecules see forexample, J. Cognard, J. Molec. Cryst. Liq. Cryst. Suppl. Ser. 1, 1(1982).

[0010] A particularly attractive technique is photoalignment which isless prone to mechanical damage than rubbing. A photosensitive polymeris polymerized by exposure with linearly polarized light. The plane ofpolarization of the light defines a preferential orientation of thechains of the photosensitive polymer. Such layers can be used asalignment layers for a broad range of polymer and small molecule liquidcrystals (M. Schadt, et al., Nature 381, 212 (1996)).

[0011] Uniaxially aligned liquid-crystalline polymers have beenincorporated as active light-emissive layers into polymer light emittingdiodes to produce linearly polarized light (M. Grell, et al., Adv. Mat.9, 798 (1998); G. Lussem, et al., Liquid Crystals 21, 903 (1996)).

[0012] EP 0786 820 A2 discloses the device structure of an organic thinfilm transistor in which the organic semiconducting layer is in contactwith an orientation film, such as a rubbed polyimide layer. Theorientation film is intended to induce alignment of the organicsemiconducting layer when the latter is deposited on top of theorientation film. However, for most organic semiconducting materials, inparticular for conjugated polymers processed from solution, meredeposition onto an orientation film is not sufficient to inducealignment in the organic semiconductor.

[0013] WO99/10929 and WO99/10939 disclose a method of forming a polymerfield-effect transistor involving building up a cross-linked layerstructure and a method of forming an interconnect in such a structure.Each layer is converted into an insoluble form prior to solutiondeposition of the next layer.

[0014] According to one aspect of the present invention there isprovided a method for forming an electronic device having asemiconducting active layer comprising a polymer, the method comprisingaligning the chains of the polymer parallel to each other by bringingthe polymer into a liquid-crystalline phase. This aspect of theinvention also provides an electronic device formed by such a method.

[0015] According to a second aspect of the present invention there isprovided an electronic device having a semiconducting active layer inwhich the polymer chains have been aligned parallel to each other bybringing the polymer into a liquid-crystalline phase. Preferably thechains are aligned parallel to each other.

[0016] The alignment of the chains may suitably be referred to asuniaxial alignment since at least within a localised domain oforientation, and more preferably over a wider extent, the parallelalignment of the polymer chains indicates a single axis of alignment.

[0017] The electronic device is suitably a switching device. Theelectronic device is preferably a transistor, most preferably athin-film transistor. The device may thus be a polymer transistor.

[0018] The said liquid-crystalline phase may be a nematic phase or asmectic phase.

[0019] The step of bringing the polymer into the liquid-crystallinephase suitably comprises heating the polymer. There is preferably asubsequent step of cooling the polymer to fix its structure. Thatcooling is preferably sufficiently rapid that the polymer retains thesaid alignment in a preferred uniaxial direction after the cooling. Thecooling may be sufficiently rapid that the polymer is in an amorphous,glassy state after the cooling. The cooling may involve quenching thepolymer. The cooling is preferably from above the glass transitiontemperature of the polymer. The cooling is conveniently to ambienttemperature, for example room temperature (20° C.).

[0020] The said method may comprise forming source and drain electrodesof the transistor in locations relative to the active layer such thatthe channel of the transistor is oriented parallel to the alignmentdirection of the polymer chains. Accordingly, the said device may have achannel that is oriented parallel to the alignment direction of thepolymer chains.

[0021] The method preferably comprises depositing the polymer on top ofan alignment layer capable of inducing the said alignment of thepolymer. The method preferably comprises the step of forming thealignment layer, for example by mechanical rubbing of a substrate.

[0022] Preferably the parallel alignment of the polymer chains extendsover a distance/domain size of at least 100 nm and more preferably atleast 1 μm, most preferably at least 10 μm.

[0023] It is preferred that the polymer chains have monodomain, uniaxialalignment over the area of the electronic device. However, performanceimprovements may already be obtained if the alignment occurs onlylocally, that is, if the polymer is in a multidomain configuration withseveral domains with randomly oriented directors located within theactive area of the device. In each domain the polymer chains would bealigned uniaxially parallel to the director, when brought into the LCphase. To produce films in a multilayer configuration no alignment layeris needed.

[0024] The polymer may be a semiconducting polymer. The polymer may be arigid-rod liquid-crystalline polymer. The polymer may be a conjugatedpolymer. The polymer may be a polyfluorene polymer, for example apolyfluorene homo-polymer or a polyfluorene based block copolymer. Thepolymer may, for example be F8 or F8T2.

[0025] The semiconducting polymer may suitably be deposited fromsolution. It is preferred that it is soluble in a non-polar organicsolvent, but is insoluble in a polar solvent.

[0026] The method may also comprise the step of forming an activeinterface of the transistor by solution deposition of a second polymerlayer. That second polymer layer may be deposited on top of asolution-processed polymer layer that has not been converted into aninsoluble form prior to the deposition of the second polymer layer. Thesolution-processed layer may be the aforesaid aligned layer and/orsemiconductor active layer.

[0027] The second layer may provide a gate insulator of the transistor.The second layer may be deposited from a polar organic solvent in whichthe said solution-processed polymer layer is not soluble. The solutiondeposition of the second polymer layer is preferably performed after thesaid alignment step. The second layer may be soluble in an alcoholsolvent, such as isopropanol or butanol. It may comprise polyvinylphenol(PVP).

[0028] An aspect of the present invention also provides a logic circuitcomprising a transistor as set out above. Such a logic circuit may alsoinclude at least one optical device. An aspect of the present inventionalso provides an active matrix display comprising a transistor as setout above, for example as part of voltage hold circuitry of a pixel ofthe display.

[0029] According to a further aspect of the present invention there isprovided a method for forming an electronic device (for example atransistor) comprising the step of forming an active interface of thedevice by solution deposition of a polymer layer directly on top of asolution-processed polymer layer that has not been converted into aninsoluble form prior to the deposition of the second polymer layer.

[0030] According to a further aspect of the invention there is provideda method for forming an electronic device having a semiconducting activelayer comprising a polymer, the method comprising inducing parallelalignment in the chains of the polymer by bringing the polymer into aliquid-crystalline phase.

[0031] According to a further aspect of the invention there is provideda method for forming an electronic device having a semiconducting activelayer comprising a polymer, the method comprising the step of bringingthe polymer into a liquid-crystalline phase.

[0032] According to a further aspect of the invention there is provideda method for forming an electronic device having a semiconducting activelayer comprising a polymer, the method comprising aligning the chains ofthe polymer within domains by bringing the polymer into aliquid-crystalline phase.

[0033] A method for forming an electronic device having a semiconductingactive layer comprising a polymer, the method comprising aligning thechains of the polymer as a monodomain oriented in a preferred uniaxialdirection within the layer of the electronic device by bringing it intoa liquid-crystalline phase.

[0034] Preferred aspects of the said further aspects of the inventioninclude analogously all those set out above in relation to the otheraspects of the invention.

[0035] The present invention will now be described by way of examplewith reference to the accompanying drawings, in which:

[0036]FIG. 1 is a schematic diagram of the top-gate device configurationfor LC polymer TFTs and orientation of the devices on the substrate withrespect to the rubbing direction;

[0037]FIG. 2 shows the polarized optical absorption spectra of auniaxially aligned T2/PVP TFT measured in the metal-free regions withpolarization of the light parallel and normal to the rubbing direction;

[0038]FIG. 3 shows an optical micrograph of an aligned T2/PVP TFT viewedunder crossed polarizers. The source and drain electrodes were definedby photolithography (Channel length L=20 μm);

[0039]FIG. 4 shows a schematic diagram of bottom (a) and top-gate (b)TFT configurations;

[0040]FIG. 5 shows output characteristics of a top-gate P3HT/PVP TFT ona glass substrate;

[0041]FIG. 6 shows output characteristics (a) and transfercharacteristics (b) of an aligned top-gate T2/PVP TFT (L=210 μm, channelwidth W=1.5 mm);

[0042]FIG. 7 shows saturated (a) and linear (b) transfer characteristicsof T2/PVP TFTs with channels oriented parallel and normal to the rubbingdirection (L=210 μm, W=1.5 mm);

[0043]FIG. 8 shows the structure of the discotic liquid crystal moleculehexabenzocoronene (HBC) and the desired orientation of the discoticcolumns on the alignment layer with respect to the direction of in-planeFET charge transport; and

[0044]FIG. 9 shows alternative configurations for aligned polymer TFTs.

[0045] The present method provides a means of forming uniaxially alignedpolymers suitable for use, for example, as an active semiconductor layerin an electronic device such as a transistor—especially a polymer thinfilm transistor (TFT). The method involves alignment of the polymer bymeans of a liquid crystal phase.

[0046] In a device such as a TFT formed with such polymer as the activelayer, current flow is suitably either preferentially along the polymerchains or preferentially normal to the polymer chains (FIG. 1). This canallow useful uniformity of charge transport properties in the relevantdirection.

[0047] The method is described below with specific reference topolyfluorene-based homopolymers such as poly-9,9-dioctylfluorene (F8)and block-copolymers such as poly(9,9-dioctylfluorene-co-dithiophene(F8T2). These main-chain LC polymers exhibit nematic LC phases above160° C. (F8), and 265° C. (F8T2), respectively. However, the method isnot limited to these materials or materials of those types, and isapplicable to a wide range of rigid-rod and LC polymers.

[0048] In one important preferred step of the method a polymeric gateinsulator may be deposited by solution-deposition on top of the alignedsemiconductive polymer forming an abrupt active interface between thesemiconductor and the insulator.

[0049] The fabrication of an aligned polymer FET having a top-gate thinfilm transistor (TFT) configuration will now be described. (See FIG. 1).Other devices and devices of other formations may alternatively be made.In a first step a thin (500 Å) polyimide precursor film (Merck ZLI 2630Polyimide kit) is spin-coated onto a glass substrate (7059 glass,Corning) and converted to polyimide by heating at 65° C. for 15 min and300° C. for 1 h. This polyimide precursor was selected for its highthermal stability and glass transition temperature providing goodalignment capability at elevated temperature. Other materials may beused. Then the polyimide film is rubbed mechanically with a nylon clothmounted on a mechanical drum to allow it to act as an alignment layer aswill be described below. A suitable procedure for forming such analignment layer is described in M. Grell, et al., Adv. Mat. 9, 798(1998). Care is preferably taken to minimize the particle contaminationduring the rubbing process as particles are believed to be primarilyresponsible for device failure and gate leakage in finished devices

[0050] Other techniques to fabricate the alignment layer could be usedas well instead of the above technique. A particularly attractivetechnique is photoalignment, as it does not involve any mechanicaltreatments of the alignment layer that may cause mechanical damage tothe film and reduce the yield of devices (M. Schadt, et al., Nature 381,212 (1996).

[0051] Gold source-drain electrodes are then defined on thepolyimide/glass substrate either by evaporation through a shadow mask orby conventional photolithography. For the photolithographic patterningof the gold electrodes lift-off techniques are preferred. Direct etchingof the gold film has been found to adversely affect the alignmentcapability of the underlying polyimide presumably by modification of thepolyimide surface when exposed to the etching solution (aqua regia).With lift-off-techniques in which the surface of the polyimide is onlyexposed to the acetone solvent with which the photoresist and the goldfilm is lifted-off, the alignment of the LC polymers between the sourceand drain electrodes (FIG. 3) was as high as on plain substrates withoutsource-drain electrodes.

[0052] Other deposition and patterning of the electrodes such as directink-jet printing of a conducting polymer such aspolyethylenedioxythiophene doped with polystyrene sulfonate (PEDOT/PSS)may be used. On the same substrates devices are defined in which the TFTchannel is either parallel or perpendicular to the rubbing direction.

[0053] As the next step the LC semiconducting polyfluorene polymer isdeposited by spin-coating from a 1 weight % solution in mixed xylenes.The thickness of the polymer film is on the order of 150-1000 Å. Thinpolymer films are preferred to minimize contact resistance effectscaused by transport of charge carriers through the bulk of thesemiconducting film from the source-drain contacts to the activeinterface. The polymer film is then heated into its LC phase at 200° C.for 24-48 h (F8) and 280° C. for 1-15 minutes (F8T2), respectively.During the annealing the polymer aligns on the surface of the rubbedpolyimide surface. The films are then brought into a glassy state byrapid quenching to room temperature. The quenching is performed byquickly moving the substrates from the hot stage onto a metallic surfaceat room temperature. The quenching step is thought to preserve theuniaxial alignment of the LC phase and to suppress crystallization andformation of grain boundaries that would form if the films were cooledslowly through the phase transition between the LC and the crystallinephase. Crystalline grain boundaries may act as charge carrier traps andadversely affect the transistor performance. During the annealing stepcare is taken not to contaminate the surface of the polymer film, byperforming the annealing steps either in vacuum or under inertatmosphere.

[0054] To further enhance the structural order in the polymer film andthe degree of uniaxial alignment additional annealing steps at lowertemperatures may be performed. The sample may also be kept in asaturated solvent atmosphere at elevated temperatures after thealignment.

[0055]FIG. 2 shows optical absorption spectra measured in the metal-freeregions of the completed TFT substrate. The absorption between 2.5-3 eVbelongs to the T2 polymer. The absorption is stronger for lightpolarized parallel to the rubbing direction than for light polarizedperpendicular to it. Since this optical transition is polarized alongthe polymer chain we conclude that the polymer chains are alignedparallel to the rubbing direction. The dichroic ratio estimated from theabsorption spectra is ≈9.6, which is a measure of the high degree ofuniaxial alignment of the polymer film.

[0056]FIG. 3 shows an optical micrograph of the channel region of acompleted TFT device with a uniaxially aligned T2 layer. The image isviewed in reflection mode through the glass substrate. Under crossedpolarizers the interdigitated gold source-drain electrodes appear dark.If the polarizers make a 45° angle with the rubbing direction s(parallel to the channel length L) the plane of polarization of theincident light is rotated when passing through the uniaxially alignedpolymer film and some of the reflected light can pass through the secondpolarizer. As a consequence the film appears bright in between the darksource drain electrodes (FIG. 3a). However, if one of the crossedpolarizers is oriented along the rubbing direction no rotation of thepolarization of the incident light can occur and the channel regionappears dark as well (FIG. 3b). In FIG. 3b the intensity of the incidentlight has been enhanced relative to FIG. 3a to make the contrast betweenthe Au fingers and the channel region visible. These observationsclearly show that the polymer chains are aligned uniaxially in thechannel of the TFTs.

[0057] After the processing and annealing steps of the alignment of theactive semiconducting polymer the TFT devices are completed bysolution-deposition of a gate insulating layer and a metallic gateelectrode on top. In order to allow the fabrication of the transistorchannel on the top surface of the aligned LC polymer film some criticalproblems had to be solved. For the formation of a device having optimalelectrical properties it is greatly preferred (a) that the underlyinglayers are not dissolved nor swelled by the solvents used for thedeposition of the gate insulator and (b) that at the same time thewetting properties of the solutions on the underlying layers allow thedeposition of smooth and continuous insulating films.

[0058] It should be emphasized that preferred feature (a) isparticularly critical since the current flow in a TFT is confined to atypically 10 nm thick interfacial layer at the interface between theactive semiconductor and the gate insulator, where the accumulationlayer is formed. The performance of the TFT is determined by thestructural and electronic properties of this interfacial layer, whereasthe properties of the bulk of the semiconductor layer are of secondaryimportance only. The performance of the TFT is very sensitive to thestructural properties and abruptness of the active interface. Anyintermixing between the semiconductor and the insulator during thesolution deposition will result in high interfacial roughness anddegradation of the electronic properties of the accumulation layer.

[0059] Preferred feature (b) implies that the gate-insulating layerneeds to be smooth and continuous without pinholes and have a highdielectric strength in order to allow the application of a high electricfield to form the accumulation layer.

[0060] The above mentioned preferred features (a) and (b) are not onlyrelevant for fabricating liquid-crystalline polymer TFTs, asdemonstrated here, but apply in general to solution processed,all-polymer TFTs with top-gate structure (FIG. 4b), and analogously todevices with the more conventional bottom-gate structure (FIG. 4a).

[0061] Previous approaches to fabricate all-polymer TFTs have usedprecursor routes. A soluble precursor material is solution-deposited asa first layer, and then converted into the final polymer that is notsoluble in common organic solvents. The conversion may involve theelimination of solubilizing side groups, an intrachain chemical reactionto form a more rigid polymer backbone or a cross-linking reaction. Theconversion usually involves a heat treatment and/or exposure to achemical reagent. To fabricate bottom-gate all-polymer TFTsprecursor-route insulating layers (FIG. 4a) such as polyimide (Z. Bao,et al., Chem. Mat. 9, 1299 (1997)) have been used. For top-gate devicesprecursor-route poly-thienylene-vinylene (PTV) as the activesemiconductor has been used (C. Drury, et al., APL 73, 108 (1998)).However, the use of precursor routes severely limits the choice ofsemiconducting and insulating polymers. For many polymers such ashigh-mobility poly-3-hexylthiophenes (P3HT) no suitable precursor routesare known. To the best of our knowledge all-polymer top-gate FETs with asoluble polymer as the active semiconductor have not been fabricated. Itis non-obvious and has not been demonstrated yet that a sufficientlyabrupt interface can be formed by solution-deposition techniques.

[0062] Here we demonstrate a method by which high-mobility all-polymertop-gate TFTs can be fabricated with solution-processible polymers bymaking use of the different solubility of the semiconducting andinsulating polymers in polar and non-polar solvents. We use non-polarsemiconducting conjugated polymers such as P3HT, F8, F8T2 orpoly(9,9′-dioctyl-fluorene-co-N-(4-butylphenyl) diphenylamine) (TFB)that have a low solubility in polar solvents such as dimethylformamide(DMF), Propylene-glycol-methyl-ether-acetate (pygmea), water oralcohols. We use insulating polymers such as poly-hydroxystyrene (alsocalled poly-vinylphenol (PVP)), poly-vinyl-alcohol (PVA)polymethylmethacrylate (PMMA) or poly-isobutyl-methyl-methacrylate(PiBuMA) which can be dissolved in polar solvents. PVP has been usedpreviously to fabricate all-polymer top-gate TFTs in combination with aninsoluble semiconducting polymer (PTV) fabricated by a precursor route(C. Drury, et al., APL 73, 108 (1998)).

[0063] The particular solvent/insulating polymer combination is chosenaccording to the polarity of the solvent and the wetting properties ofthe solution on the surface of the underlying semiconducting polymer.Alcohols such as methanol, ethanol, butanol, or isopropanol have beenfound to be particularly suitable. Most non-polar semiconductingpolymers have exceptionally low solubility in alcohols. This is partlybecause of the highly polar nature of the hydroxyl groups of thealcohol, partly because of hydrogen bonding among the alcohol moleculesin solution. This further lowers the solubility of those hydrophobicpolymers that cannot take part in hydrogen bonding. Solutions ofinsulating polymers such as PVP in alcohols also tend to exhibit arelatively high viscosity. This is believed to be partly due to thehydrogen bonding between alcohol solvent molecules and the hydroxylgroups of the PVP polymer in solution. A moderately high viscosityfacilitates the deposition of continuous smooth films helping toovercome the wetting problems of a polar solution on a non-polarsubstrate. Alcohols with different boiling points can be used. Filmsfabricated from high boiling point alcohols such as butanol tend to havesmoother film morphology than those prepared from low boiling alcoholssuch as isopropanol or methanol. However, no significant differences ofTFT mobility were observed for devices fabricated with the same gateinsulating polymer deposited from different alcohol solvents. Thisindicates that the integrity and abruptness of the active interface iswell preserved with alcohol solvents.

[0064] Other polar solvents such as Poly-propylene-glycol-ether-acetate(pygmea) have been used successfully as well. Although the devicesfunctioned reasonably well, the TFT mobility was typically a factor of 2lower than with alcohol solvents. This is attributed to some smallresidual solubility of the polyfluorene polymer in the polar pygmeasolvent. To test this hypothesis F8T2 films were immersed in pygmea forseveral minutes. Small changes of the optical absorption spectrum of thefilm were observed afterwards, indicating that the solvent interactedwith the surface of the F8T2 film (either by dissolution or swelling).No such changes were observed in the case of alcohol solvents.

[0065] With PVP gate insulators deposited from alcohol solutions TFTswere fabricated with a performance comparable to that of control devicesfabricated on standard SiO₂/Si FET substrates. Before deposition of thePVP the substrate is coated by the same alcohol solvent that is used forthe PVP solution and spun-dried. This step is intended to wash off anyalcohol-soluble residual components of the polymer film. The PVP gateinsulator is then spin-coated from a 5-10 weight % solution, preferablyin an alcohol such as isopropanol or butanol. The capacitance of 1.2 μmthick PVP films are on the order of 3-5 nF/cm² as measured by impedancespectroscopy on metal-insulator-semiconductor (MIS) diodes, and plaincapacitor structures. Thinner PVP gate insulators with thicknesses of200-500 nm are preferred and have been fabricated as well. The devicesare completed by deposition of a gold top-gate electrode through ashadow mask.

[0066] The source-drain and gate electrodes of the TFT may also befabricated from a conducting polymer that is patterned by a suitabletechnique such as ink-jet printing, soft-lithographic patterning orscreen printing.

[0067]FIG. 5 shows the output characteristics of a top-gate P3HT/PVP TFTfabricated on a standard glass substrate (without the polyimidealignment layer). The device characteristics are comparable to that ofdevices fabricated in the same deposition run on conventional TFTsubstrates with bottom-gate SiO₂ insulators. The mobility extracted fromthe transfer characteristics in the saturation regime is on the order of0.01-0.02 cm²Vs. The OFF-conductivity of the device is somewhat higherthan that of the corresponding bottom-gate devices which is believed tobe due to doping of the surface of the P3HT surface by residual oxygen.This is not related to the specific device structure and could be solvedby exposing P3HT to a reductive dedoping treatment in hydrazine prior tothe deposition of PVP (H. Sirringhaus, et al., Science 280, 1741(1998)).

[0068] Devices were also fabricated with F8T2 as the semiconductingpolymer. On a plain glass substrate, that is without an alignment layer,both the mobility of 0.003-0.005 cm²/Vs and the ON-OFF current ratio arecomparable to those of bottom-gate transistors fabricated on SiO₂.

[0069] These results demonstrate clearly that it is possible tofabricate abrupt interfaces between two solution-processed polymerlayers and maintain the high degree of interfacial order which isrequired to obtain high charge carrier mobilities of >10⁻³-10⁻² cm/Vs inthe accumulation layer of a TFT.

[0070] The procedure to deposit a polymeric gate insulator from a polarsolvent has been applied to fabricate top-gate TFT devices on top ofuniaxially aligned LC polymer films. FIG. 6 shows the output (a) andtransfer characteristics (b) of an aligned T2/PVP TFT with the channeloriented parallel to the rubbing direction. The device characteristicsshow good current saturation and ON-OFF current ratios >10⁴-10⁵. FIG. 7compares the transfer characteristics in the saturation (a) and linear(b) regime of aligned T2/PVP TFT devices with channels parallel andperpendicular to the rubbing direction s. The devices are fabricated onthe same substrate. The transistor current is higher by a factor oftypically 5-8 if the current flow is along the rubbing direction, thatis, along the preferential direction of the polymer backbones. Themobilities extracted from the transfer characteristics are 0.009 -0.02cm²/Vs for transport parallel to the direction of preferential alignmentof the polymer chains and 0.001-0.002 cm²/Vs for the perpendiculardirection.

[0071] These mobility values can be compared to those of isotropic TFTdevices, that underwent the same thermal treatment as the uniaxiallyaligned devices, but exhibit no monodomain alignment. These isotropic,multidomain devices (x) are prepared on areas of the substrate that arenot coated by polyimide, and typically exhibit mobilitiesμ_(x)=0.003-0.005 cm²/Vs with μ_(⊥)<μ_(x)<μ_(∥).

[0072] This demonstrates that uniaxial alignment results in asignificant improvement of the mobility by typically a factor of 3compared to isotropic devices. However, even the isotropic devicesexhibit significantly higher mobilities and better turn-on voltagestability than devices in which the F8T2 film underwent no thermaltreatment (μ_(as-spun)<10⁻³ cm²/Vs). In the multidomain, isotropicdevices the domain sizes are on the order of 0.1-1 μm, as estimated fromoptical micrographs, that is, the TFT channel contains several LC domainboundaries. This indicates that LC domain boundaries in the nematicglass do not act as carrier traps to the same extent as microcrystallinegrain boundaries in as-spun films.

[0073] The observation of a significant mobility anisotropy shows thatwith the device configuration demonstrated here it is possible toexploit fast intrachain transport along the polymer chain usingconjugated polymers that form LC phases and can be aligned by usingalignment layers. Further optimisation of the alignment process, whichwill include optimisation of the annealing and rubbing procedure andtreatment of the aligned films with solvent vapours is likely to resultin a further increase of the dichroic ratio in the channel region andhigher mobilities.

[0074] Other LC polymers may be used as well. Higher mobilities areexpected in materials that show stronger interchain interactions, andfacilitate interchain hopping of charge carriers between adjacentpolymer chains. The most desirable orientation to obtain high mobilitieswould be a structure in which the direction of π-π stacking waspreferentially oriented in the plane of the film perpendicular to therubbing direction. This would require a biaxial liquid crystallinepolymer showing an anisotropy and preferred orientation of the moleculesin a plane normal to the alignment direction of the polymer backbone (A.M. Donald, A. H. Windle, Liquid Crystalline Polymers, Cambridge SolidState Science Series, ed. R. W. Cahn, E. A. Davis, I. M. Ward, CambridgeUniversity Press, Cambridge, UK (1992)).

[0075] LC polymers that yield a higher degree of alignment and longerpersistence length along the polymer chain (M. Grell et al.,Macromolecules 32, 5810 (1999)) are also expected to yield highermobilities.

[0076] In order to allow for efficient hole injection from thesource-drain electrodes the LC polymer should have a sufficiently lowionisation potential, preferably below 5.5 eV, that is well matched tothe work function of common source-drain electrode materials such asinorganic metal electrodes (gold, platinum, aluminium, etc.) orconducting polymers such as PEDOT.

[0077] Other types of LC molecules to which this TFT fabrication processis applicable are disc-shaped conjugated molecules with flexible sidechains forming discotic mesophases. Along the 1 D columnar stacks highcharge carrier mobilities can be obtained (D. Adam, et al. Nature 371,141 (1994)). In some discotic molecules such as hexabenzocoronenes(HBC)) (P. Herwig, et al., Adv. Mater. 8, 510 (1996)) the columns tendto be oriented in the plane of the film such that the high mobilitiesalong the columns could be exploited in in-plane transistor transport(FIG. 8). HBC has a low solubility in polar solvents as well. Techniquesto align discotic molecules with the use of alignment layers have beendeveloped (Mori, Hiroyuki, European patent application 94116645.6;Kamada, et al., European patent application 94114956.9).

[0078] Several modifications of the device configuration are possible.The source/drain electrodes may be deposited after deposition/andalignment of the LC polymer (FIG. 9a). This will facilitate chargecarrier injection at the source-drain electrodes, and improve thealignment of the polymer in the vicinity of the electrodes. Thealignment layer may be used as the gate insulator itself in aconventional bottom gate configuration either in a single-layer (FIG.9c) or double-layer configuration (FIG. 9b). In the latter the secondinsulating layer provides additional dielectric strength. The bottomgate structure may be less suitable for alignment layers produced bymechanical rubbing due to defects at the active interface, but it isbelieved to be suitable for other alignment techniques such asphotoalignment.

[0079] Applications of polymer TFTs according to this invention are inpolymer TFT logic circuits (C. Drury, et al., APL 73, 108 (1998)) or aspixel drive transistors in high-resolution, active matrix displays (H.Sirringhaus, et al., Science 280, 1741 (1998)). Examples of suchdisplays are active matrix polymer LED displays, liquid-crystal displays(LCD) or electrophoretic displays. The enhanced charge carrier mobilityalong the direction of preferential uniaxial alignment of the polymerchains compared to the mobility of the isotropic polymer film can beused to increase the operation speed and the drive current capability ofthe TFTs.

[0080] Some preferred aspects of selection of semiconducting polymersfor use as described above will now be described.

[0081] The polymer should preferably be able to form monodomains on thealignment layer at temperatures below 300° C., most preferably below200/150° C. To be able to inject charge carriers and to obtain stableTFT characteristics the ionisation potential of the material shouldpreferably be below 5.8 eV, preferably below 5.5 eV, most preferablybelow 5.1 eV.

[0082] The polymer needs preferably to have good stability againstchemical reaction with the atmosphere (oxygen, water) etc during thehigh temperature annealing step. The polymer should have an ionisationpotential larger than 4.9 eV, preferably higher than 5.1 eV. The TFTshould have a ON-OFF current switching ratio larger than 10{circumflexover ( )}3, most preferably larger than 10{circumflex over ( )}4, and aturn-on gate voltage V0 less than −30V, most preferably less than −10V.

[0083] A preferred class of materials to achieve good environmentalstability and high mobility are A-B rigid-rod block-copolymerscontaining a regular ordered sequence of A and B blocks. Suitable Ablocks are structurally well defined, ladder type moieties with a highband gap (e.g. greater than 2.5 eV), that have high ionisationpotentials larger than 5.5 eV as a homopolymer and good environmentalstability. These features are preferable independently and incombination. Examples of suitable A blocks are fluorene derivatives (forexample those disclosed in U.S. Pat. No. 5,777,070), indenofluorenederivatives, or phenylene or ladder-type phenylene derivatives (forexample those disclosed in J. Grimme et al., Adv. Mat. 7, 292 (1995)).Suitable B blocks are hole-transporting moieties with lower bandgaps(for example less than 2.5 eV) that contain heteroatoms such as sulphuror nitrogen, and as a homopolymer have ionisation potentials less than5.5 eV. These features are preferable independently and in combination.

[0084] Examples of hole-transporting B blocks are thiophene derivatives,or triarylamine derivatives. The effect of the B block is to lower theionisation potential of the block copolymer. The ionisation potential ofthe block copolymer is preferably in the range of 4.9 eV≦I_(p)≦5.5 eV.Examples of such copolymers are F8T2 (ionisation potential 5.5 eV) orTFB (U.S. Pat. No. 5,777,070).

[0085] Instead of hole transporting semiconducting polymers solubleelectron transporting materials may be used as well. These require ahigh electron affinity larger than 3 eV, preferably larger than 3.5 eV,to prevent residual atmospheric impurities such as oxygen to act ascarrier traps. AB-type block copolymers with a structurally-welldefined, ladder-type A block with a high ionisation potential largerthan 5.5 eV and an electron-transporting B block that increases theelectron affinity of the copolymer to a value higher than 3 eV,preferably higher than 3.5 eV are preferred. Examples of A blocks arefluorene derivatives (for example those disclosed in U.S. Pat. No.5,777,070), indenofluorene derivatives, phenylene or ladder-typephenylene derivatives (for example those disclosed in J. Grimme et al.,Adv. Mat. 7, 292 (1995)). Examples of electron-transporting B blocks arebenzothiadiazole derivatives (for example those disclosed in U.S. Pat.No. 5,777,070), perylene derivatives, naphtalenetetracarboxylic diimidederivatives or fluorinated thiophene derivatives.

[0086] Many of these block copolymers exhibit liquid-crystalline phasesat elevated temperatures (F8, F8T2), whereas others such as TFB do notform LC phases. With TFB field-effect mobilities of 0.002 cm²/Vs havebeen obtained in the top-gate device configuration. In the case of TFBan annealing step at a temperature of 200-290° C., i.e., above the glasstransition temperature, followed by rapid quenching has been found toenhance the mobility and threshold voltage stability. Although in thecase of TFB no LC transition could be detected, this improvement oftop-gate TFB devices is attributed to a reduction of charge trapping atstructural defects such as crystalline domain boundaries when thepolymer is prepared in an amorphous glassy state.

[0087] In general, polyfluorene-based block-copolymers are a promisingnew class of conjugated polymers that have not hitherto been used inpolymer transistor devices. For the reasons indicated above,polyfluorene-based block copolymers are one preferred class of materialsfor aligned semiconductor layers of devices described above.

[0088] The present invention may include any feature or combination offeatures disclosed herein either implicitly or explicitly or anygeneralisation thereof, irrespective of whether it relates to thepresently claimed invention. In view of the foregoing description itwill be evident to a person skilled in the art that variousmodifications may be made within the scope of the invention.

1. A method for forming an electronic device having a semiconductingactive layer comprising a disc-shaped molecular material, the methodcomprising aligning columns of the disc-shaped molecules parallel toeach other by bringing the disc-shaped molecular material into a liquidcrystalline discotic phase.
 2. A method as claimed in claim 1, whereinthe device is a transistor.
 3. A method as claimed in claim 1 whereinthe step of bringing the disc-shaped molecular material into theliquid-crystalline phase comprises heating the disc-shaped molecularmaterial.
 4. A method as claimed in claim 3, comprising the step ofquenching the disc-shaped molecular material subsequent to the saidheating.
 5. A method as claimed in claim 4, wherein the said quenchingstep is such as to form the disc-shaped molecular material into anamorphous glassy state.
 6. A method as claimed in claim 1, comprisingforming source and drain electrodes of the transistor in locationsrelative to the active layer such that the channel of the transistor isoriented parallel to the alignment direction of the columns ofmolecules.
 7. A method as claimed in claim 1, comprising depositing thedisc-shaped molecular material on top of an alignment layer capable ofinducing the said alignment of the columns of molecules.
 8. A method asclaimed in claim 7, comprising the step of forming the alignment layerby mechanical rubbing of a substrate.
 9. A method as claimed in claim 1,wherein the disc-shaped molecules are conjugated molecules.
 10. A methodas claimed in claim 1, comprising the step of depositing the disc-shapedmolecular material from solution.
 11. A method as claimed in claim 1,comprising the step of forming an active interface of the transistor bysolution deposition of a dielectric polymer layer on top of thedisc-shaped molecular material.
 12. A method as claimed in claim 1,wherein the columns of molecules are arranged in uniaxial alignment. 13.A method as claimed in claim 1, wherein the columns of molecules arearranged in uniaxial, monodomain alignment.
 14. A method as claimed inclaim 1, wherein the columns of molecules are aligned in domains oflocal parallel alignment.
 15. An electronic device formed by the methodof claim
 1. 16. A logic circuit comprising a transistor as claimed inclaim
 15. 17. A logic circuit as claimed in claim 16 including at leastone optical device.
 18. An active matrix display comprising a transistoras claimed in claim
 15. 19. An electronic device having a semiconductingactive layer comprising a disc-shaped molecular material in whichcolumns of disc-shaped molecules have been aligned parallel to eachother by bringing the disc-shaped molecular material into aliquid-crystalline phase.
 20. An electronic device as claimed in claim19, wherein the device is a transistor.
 21. An electronic device asclaimed in claim 19, wherein the device is a thin-film transistor. 22.An electronic device as claimed in claim 20, wherein the channel of thetransistor is oriented substantially parallel to the direction of thealigned columns of molecules.
 23. An electronic device as claimed inclaim 19, comprising an alignment layer directly underlying the activelayer.
 24. An electronic device as claimed in claim 22, wherein thealigned columns of molecules are semiconducting columns of molecules.25. An electronic device as claimed in claim 24, wherein the alignedcolumns of molecules are in an amorphous glassy state.
 26. A method forforming an electronic device having a semiconducting active layercomprising a disc-shaped molecular material, the method comprisingaligning columns of the disc-shaped molecules within domains by bringingthe disc-shaped molecular material into a liquid-crystalline phase. 27.A method for forming an electronic device having a semiconducting activelayer comprising a disc-shaped molecular material, the method comprisingaligning columns of the disc-shaped molecules as a monodomain orientedin a preferred uniaxial direction within the layer of the electronicdevice by bringing the disc-shaped molecular material into aliquid-crystalline phase.
 28. A transistor device comprising adisc-shaped molecular material having a discotic liquid crystallinephase.